A couple of years ago I gave away a nice plump rebutia to a friend. Not long ago she said how well it had done. I was horrified, it looked exactly like the photograph above!

The rebutia had been kept on a poorly lit windowledge and yours seems to have had similar conditions, extreme shortage of good light.

Find the brightest place you can for this plant. If you have an upstairs, the light is usually better. The treatment suggested by Phil is what I would go for and no water or feed until April, in fact leave off the feed. Otherwise, and I hate to follow this path, ditch it and start again.